... is what happens when a country re-opens. China, Italy, et. al. who are further down the curve are not seeing much evidence that COVID survivors are being re-infected. There are some cases where people who were thought to have recovered coming down with it again but the medicos think it is more a case the patients didn't clear the virus (or the test applied at the time was faulty). You are correct that we don't know how long that immunity will last. This virus is novel so all bets are off in the long term. Still need effective treatments and a vaccine.